1. Field of the Invention
The invention relates to a motor provided in a steering system, and more particularly, to a steering system motor in which a starter of the motor is divided into a plurality of systems that are electrically independent from one another.
2. Description of the Related Art
Examples of a steering system motor include a motor, which serves as a power source for assisting with steering, and a motor, which serves as a power source for steering wheels. A vehicle is typically provided with a system for assisting with steering, i.e., a power steering system. The motor of a power steering system is sometimes an electromagnetic motor that is mounted, for example, near the steering column or a rack-and-pinion mechanism. Also, in a steer-by-wire type steering system, in which the wheels are steered by controlling a power source of a steering portion, the operating force that is input to an operating member, such as a steering wheel, is not transmitted to the steering portion, instead a motor that serves as the power source of the steering portion is provided for steering the wheels. In addition, a motor that applies an appropriate amount of resistance may also provided on the operating member.
The steering system motor described above greatly affects the steering so preferably it incorporates a fail-safe. For example, Japanese Patent Application Publication No. JP-A-2005-237068 describes an electromagnetic motor in which the starter is divided into two systems so that if one system fails, the wheels can be steered with the remaining system. The starter of this electromagnetic motor is symmetrically divided on a plane into two. When the system on one side fails, magnetism is generated by the other system on the other side, which causes the rotor to vibrate. This vibration alerts the driver that the motor has failed. As a result, it is expected that the driver will refrain from making any sudden steering operations and promptly have the motor repaired. In addition, Japanese Patent Application Publication No. JP-A-7-222419 describes a related electromagnetic motor having a single system starter.
With the electromagnetic motor described in JP-A-7-222419, if the single system starter fails, steering assist is of course disabled, i.e., the wheels are no longer able to be steered by the motor. Thus, the motor is not considered very reliable. In comparison, with the electromagnetic motor described in JP-A-2005-237068, even if one system fails the motor can still operate, at which time a relatively large vibration is generated. However, because it is difficult to accurately synchronize the power of the two systems, a relatively small vibration tends to be generated even when both systems are operating normally. This problem may make it difficult to enhance the practical use of a conventional steering system motor. As a result, there is still room for improvement from various standpoints with respect to a steering system motor.